Package strategies for net40 and sl4 in same solution

Just wondering what strategies are being used to package Solutions with a mix of projects that target the full framework and SL?

I'd like to publish one package with multi-targetting. Solution structure:
solution.sln
/a/alibrary.csproj
/alibrary.silverlight/alibrary.silverlight.csproj

This seems to rule out generating the package from the project folders from (csproj + nuspec). Is the approach to copy assemblies to the convention based folder structure and create the package from the binaries?
lib
/net40/alibrary.dll
/sl4/alibrary.silverlight.dll

The drawback with this approach is double handling of dependencies. Suppose /a/alibrary.csproj has a third-party dependency added via NuGet. If you're packaging from the NuGet convention folder structure above, you would have added the NuGet dependency twice?
One time with the solution open in Visual Studio, and then again to the nuspec used for the package?

Currently, the feature that packs from a csproj is not as flexible as it should be, and won't cover this scenario. We should improve it, but for now your best best is to create your nuspec without relying on the csproj.